Ring-wing aircraft

ABSTRACT

A ring-wing aircraft suited particularly, although not exclusively, to use in micro-unmanned air vehicles (UAV&#39;s) with ring-wings. An aircraft ( 10 ) according to the invention comprises a ring-wing ( 11 ) defining a duct ( 16 ) with a longitudinally-extending central axis ( 31 ), propulsion element ( 15 ) located within the duct and moveable aerofoils ( 13, 18 ) for controlling the aircraft in flight, the ring-wing being truncated obliquely at one end, that end being the rear ( 11   b ) when in horizontal flight, to form a ring-wing with opposed sides of unequal length. This arrangement produces center of mass offset from the central axis of the ring-wing, the pendulum effect will ensure that the aircraft will roll so that its center of mass will always be at the lowest height possible when the aircraft is airborne. Therefore the aircraft has a preferred orientation, and the control surfaces can be oriented with respect to this preferred orientation. In addition, the oblique truncation at the rear keeps the center of mass towards the front of the aircraft thereby giving improved stability in all three axes.

BACKGROUND AND SUMMARY OF THE INVENTION

This application claims the priority of British patent document0014064.0, filed Jun. 10, 2000 (PCT International Application No.PCT/GB01/01769), the disclosure of which is expressly incorporated byreference herein.

This invention relates to ring-wing aircraft and is suited particularly,although not exclusively, to use in micro-unmanned air vehicles (UAV's)incorporating ring-wings. However, it will be appreciated that theinvention is equally well suited to use in ring-wing aircraft of anysize, whether they be manned or unmanned.

Unmanned air vehicles have found a number of applications, where theyhave been used to carry a wide variety of payloads. In the absence of apilot, the aircraft may be scaled down, typically to a size of 150 mm orless. The reduction in size and the absence of extra load in the form ofa pilot fit in well with the constant drive to produce lighter aircraftbecause of their inherent efficiency advantages.

In terms of flexibility it is advantageous for UAV's to be able both tohover and to fly horizontally at high speed. This criterion has led tothe development of ring-wing aircraft, i.e. an aircraft having apropeller or other propulsion system mounted within a duct defined by aring-shaped wing of substantially circular cross-section. A ring-wingaircraft is disclosed in U.S. Pat. No. 5,295,643 to Hughes MissileSystems Company, although the scale of the aircraft is not disclosed.The Hughes aircraft has a ‘toroidal’ ring-wing that defines a duct(although the ring-wing is generally flat in cross-section so that itcan be considered as essentially an annular cylinder). A propeller,stators and vanes are located within the duct. In the Hughes aircraft,the stators serve to straighten the swirling air produced by therotating propeller and hence help to balance the torque applied to theaircraft by the rotating propeller. The vanes are used as controlsurfaces for controlling the pitch and yaw of the aircraft whenairborne.

The use of a ring-wing has benefits that include reducing the noisesignature of the propeller and providing a protective housing for thepropeller that guards against both damage to the propeller and damage toanything or anyone that may otherwise come into contact with thepropeller. However, the main advantage of the ring-wing design is thatit can be used both for hovering, by orienting the propellerhorizontally, and for horizontal flight by orienting the propellervertically. In the Hughes aircraft, the transition from vertical tohorizontal flight is effected by use of the vanes.

However, the Hughes Patent does not discuss any aspects of rollpertinent to their aircraft, nor does it contain any detailed discussionon any aspects of yaw or pitch. Clearly these are important aspects onthe design of a ring-wing UAV as these aircraft must be controlledeither autonomously or by remote control, and so control and stabilityof the aircraft must be enhanced, say, over a manned vehicle where thepilot will have direct feedback from his sense of balance as to theattitude of the aircraft. Where autopilots are used, natural stabilityof the UAV is highly important as it reduces the complexity, size and,critically, the mass of the autopilot system to be installed on theaircraft.

Natural stability in the pitch and yaw planes is achieved by placing thecentre of mass of the aircraft ahead of the aerodynamic neutral pointsin the pitch and yaw planes. The shape of the ring-wing means that theaerodynamic pressures acting on all portions of the ring-wing have aline of action that inherently passes through the centre of the ring.The symmetry of the continuous ring-wing ensures that these pressuresgive rise to no contribution to rolling moment. It will be appreciatedthat the ring-wing need not be continuous: the ring may be divided intotwo or more sections and there will still be no rolling momentcontribution from the aerodynamic pressures.

However, the inherent neutral symmetry of ring-wing aircraft can beconsidered problematic. This is because any disturbance about the rollaxis leaves the aircraft in an attitude without any incrementalaerodynamic rolling moment to restore it to its undisturbedcondition—there is no tendency for the aircraft to right any inducedroll. This tendency freely to roll can lead to a requirement for aflight control system to provide a means of controlling the vehicle,thereby adding complexity, and quite often mass, to the aircraft.

An object of the invention is to overcome the disadvantages of previousdesigns of ring-wing UAV's described hereinabove.

From a first aspect, the present invention resides in an aircraftcomprising a ring-wing defining a duct with a longitudinally extendingcentral axis, propulsion means located within the duct and moveableaerofoils for controlling the aircraft in flight, the ring-wing beingtruncated obliquely at one end, that end being the rear when inhorizontal flight, to form a ring-wing with opposed sides of unequallength.

It should be noted that the term ring-wing is intended to encompassrings of substantially circular cross-section, both continuous andinterrupted, i.e. where segments of the ring have been omitted. This isbest done to leave a wing symmetrical about its central axis.

It will be appreciated that by truncating the ring-wing obliquely at itsrear produces and aircraft with a centre of mass offset from the centralaxis of the ring-wing. The pendulum effect will ensure that the centreof mass will always be at the lowest height possible when the aircraftis airborne. Hence, should the aircraft roll, the rolling moment due tothe increased height of the centre of mass will produce a rolling motionof the aircraft so that the centre of mass returns to the lowest heightpossible. It will be appreciated that the further the centre of mass isoffset from the central axis of the ring-wing, the greater the tendencyfor the aircraft to roll to correct any disturbance about its roll axis.Therefore the aircraft has a preferred orientation, and the controlsurfaces can be oriented with respect to this preferred orientation.Roll stability is thus achieved through the restoring moments providedby the offset centre of gravity rather than through aerodynamicrestoring moments.

In addition, truncating the ring-wing at its rear rather that its frontis advantageous as it produces a centre of mass ahead of the aerodynamicneutral points in the pitch and yaw planes when the aircraft is engagedin high-speed forward flight. It should be appreciated that theapplicant has realised that a simple modification of the basic ring-wingshape is highly beneficial because in addition to offsetting the centreof mass from the central axis of the ring-wing to provide self-rightingroll control, it also moves the centre of mass towards the front of theaircraft, i.e. the simple modification provides much improved stabilityin all three axes.

Optionally, the ring-wing is truncated by a planar slice through thering-wing. It is preferred that the planar slice makes an angle ofbetween 25° and 65° to the longitudinally extending central axis. It isfurther preferred for the angle to be between 25° and 45° and yetfurther preferred for the angle to be substantially equal to 45°.Optionally, the ring-wing is truncated by a curved slice, the angle madewith the ring-wing being relatively shallow at the longer side of thering-wing and relatively steep at the shorter side of the ring-wing.This arrangement conveniently produces a greater shift in the centre ofmass to the shallow end.

The payload and other components of the aircraft that do not need to belocated on the central axis of the ring-wing (e.g. remove controlreceiver, batteries or fuel tank, motor speed controller, etc.)constitute the substantial majority of the overall mass of the aircraftand, thus, should be located offset from the central axis of thering-wing. In order to maximise the pendulum effect, it is advantageousto house components and/or any payload in a compartment that is externalto the longer side of the ring-wing. Alternatively, or in addition,components and/or any payload may be housed in a compartment providedwithin the longer side of the ring-wing. Advantageously, the compartmentmay be provided at one end of the ring-wing, that end being the frontwhen in horizontal flight, as this configuration assists stability inall three axes.

The aircraft may have a pitch-controlling aerofoil component providedoffset from the central axis of the ring-wing. Optionally, it may bepositioned to reside substantially flat with the longer side of thering-wing when not deployed. Advantageously, it may be provided as anelement of the longer side of the ring-wing.

Conveniently, the aircraft may further comprise two orthogonal vaneswherein one vane provides aerodynamic lift in the same direction as thepitch-controlling aerofoil. These vanes may primarily control the rolland yaw attitude of the aircraft, and they may operate in conjunctionwith the pitch-controlling aerofoil in certain manoeuvres, to effect aturn for example.

Optionally, any aircraft defined hereinabove is unmanned. The diameterof the ring-wing may be less than 50 cm, although a diameter of lessthan 25 cm is currently preferred.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an aircraft according to a firstembodiment of the present invention;

FIG. 2 is a plan view of the aircraft of FIG. 1;

FIG. 3 is a front view of the aircraft of FIG. 1;

FIG. 4 is a section along line IV—IV of FIG. 2;

FIG. 5 is a perspective view of an aircraft according to a secondembodiment of the present invention; and

FIG. 6 is a section along line VI—VI of FIG. 5.

DETAILED DESCRIPTION OF THE DRAWINGS

As can be seen from FIGS. 1 to 3, an aircraft 10 according to a firstembodiment of the invention comprises a ring-wing 11 with a compartment12 and a rear elevator 13. The compartment 12 is located below a forwardsection 11 a of the ring-wing 11 so that it extends beyond the front ofthe ring-wing 11. The size of the aircraft can be gauged from the factthat the ring-wing 11 has a diameter of 12 cm. The ring-wing 11 isobliquely truncated at an angle α of 45° to the ring-wing'slongitudinally-extending central axis 31 to define the forward section11 a and a tail section 11 b comprising a part ring. The ring-wing 11 istruncated in planar fashion so that the tail section 11 b tapersuniformly to meet the elevator 13 at the rear of the aircraft. Thecentreline of the tail section 11 b is coincident with the centreline ofthe elevator 13.

Turning now to the front of the aircraft, a forward fairing 14 aprotrudes from the centre of a void 16 defined by the ring-wing 11. Theforward fairing 14 a is located directly in front of a four-bladedpropeller 15 which resides just inside the void 16 and rotates about thelongitudinal axis 17 of the ring-wing 11. Behind the propeller 15, arear fairing 14 b extends along the longitudinal axis to terminate abovethe tail section 11 b of the ring-wing 11. Four vanes 18 are locatedwithin the void 16 at the rear of the forward section 11 a and arearranged symmetrically around the longitudinal axis 17 of the ring-wing11 as a pair of vertical vanes 18 a and a pair of horizontal vanes 18 b.The vanes 18 protrude slightly beyond the extent of the top of theforward section 11 a of the ring-wing 11, i.e. they overlap slightlywith the tail section 11 b.

A compartment 12 is located underneath the ring-wing 11 so that it spansthe length of the forward section 11 a and protrudes forwards of theleading edge 11 c of the ring-wing 11. The compartment 12 is profiled tohave an aerodynamic shape, i.e. its outer surfaces constitute a furtherfairing.

A pair of elongate supports 19 span the void 16 at the rear of theforward section 11 a of the ring-wing 11, one horizontal and onevertical, and support the fairings 14 a, 14 b and the vanes 18.

FIG. 4 is a vertical section of the aircraft 10 and shows the aircraftin more detail. As can be seen, the compartment 12 is hollow and housesseveral components of the aircraft 10. Specifically, these are a pair ofhigh-performance batteries 20 to power the plane, a radio receiver 21 toreceive the flight control instructions transmitted by a hand-heldtransmitter used by an operator and an electronic motor speed controller22. In addition, a payload 23 is also shown. To ensure an optimalaerodynamic profile of the compartment 12, the components 20, 21, 22 andpayload 23 are arranged one in front of the other, thereby ensuring thatthe compartment 12 is shallow and narrow. The aerodynamic profile of thecompartment 12 is clearly shown in FIG. 4.

Furthermore, FIG. 4 also shows the ring-wing 11 to have a characteristiccross-sectional profile of aerofoils, thereby maximising the liftgenerated by the ring-wing 11. The aerodynamic profile of the ring-wing11 is uniform around its circumference. This typical aerofoil profile isalso used for the elevator 13 and for the vanes 18.

The propeller 15 is powered by a motor 24 housed in a cavity provided inthe rear fairing 14 b behind the propeller 15, the propeller 15 beingmounted on a central shaft of the motor 24. The body of the motor 24forms part of the sides of the rear fairing 14 b. Recessing the motor 24within the rear fairing 14 b ensures optimal aerodynamics of theaircraft 10. The supports 19 pass through the rear fairing 14 b behindthe motor 24 in through-holes sized to ensure a snug fit and terminatesnugly within holes provided in the ring-wing 11. The supports 19 arebonded securely within the holes provided in the ring-wing 11.

The vanes 18 are provided with through-holes extending across the widthof their forward end, the through-holes being sized to provide a slidingfit within the supports 19 which pass through the through-holes.Accordingly, the vanes 18 pivot about the supports 19 to provide controlsurfaces during flight. Each vane 18 is moved by an associated servomotor 25 a linked to its vane 18 by a crank arm and linkage 26 a. Theservo motors 25 a are housed within recesses 27 a of the ring-wing 11fitted with aerodynamic covers 28. Similarly, the elevator 13 is alsodeployed by a servo motor 25 b linked by a crank arm and linkage 26 b,the servo motor 25 b being housed within a covered recess 27 b of thetail section 11 b. All servo motors 25 a, 25 b draw their power from thebatteries 20, as does the propeller motor 24. Wires (not shown)distribute electricity and are routed within recesses or hollows.

The forward fairing 14 a is hollow, which is advantageous as theinternal space provides room for further payloads or, when not used as astore, reduces the overall mass of the aircraft 10.

This crucial aspect of the overall mass of the aircraft 10 drives thechoice of materials of the various components of the aircraft 10. Lowdensity materials are generally preferred. Specifically, styrofoam isused for the ring-wing 11, the compartment 12, the vanes 18, elevator 13and fairings 14 a, 14 b. Carbon-fibre composite tubes are used for thesupports 19 and also for the crank arms and linkages 26 a, 26 b incombination with spring-steel wires. Compact, low-mass servo motors 25a, 25 b and motor 24 are employed.

The careful choice of materials, component shape and positioningdescribed hereinabove results in a centre of mass 29 of the aircraft 10that is both below the central axis 17 of the ring-wing 11 and close tothe leading edge 11 c of the ring-wing 11. Therefore, the aircraft 10has considerable stability in the pitch and yaw planes and, in additionto stability about its roll axis, it is self-righting about its rollaxis.

Aspects relating to the flight of the aircraft 10 will now be described.The aircraft 10 is launched using a conventional model glider technique,namely a long length of elastic material that launches the aircraft 10into horizontal flight, the propeller 15 already running.

In level horizontal flight, the vanes 18 are not positioned exactlyhorizontally and vertically, but are slightly offset to account for thetorque produced by the rotating propeller 15 which would otherwiseproduce a tendency for the aircraft 10 to roll. Control of the aircraft10 in flight is accomplished using the elevator 13 for pitch control andthe vanes 18 for roll-control. Yaw control has been found not to benecessary—turns are accomplished by using a combination of pitch androll control in a conventional manner.

When hovering, the propeller 15 is oriented horizontally with theelevator 13 being lowermost. For this mode the elevator 13 controlspitch and the vanes 18 control both roll and yaw. Rather than beingoriented exactly upright, the aircraft 10 is tilted slightly in order tomaintain the centre of mass 29 in line with the thrust axis. For finecontrol of the thrust axis, the propeller 15 may have a conventionalhelicopter-like pitch/collective mechanism so that the angle of thepropeller blades 15 changes as they rotate from the approaching sectorto the retreating sector. Alternatively, this fine control could beaccomplished by mounting the motor 24 on gimbals thereby allowing thewhole axis of rotation of the propeller 15 to be tilted. The aircraft 10can be moved around at low speed in the hover mode by slight alterationof the tilt of the aircraft 10.

Transition between horizontal flight and hover is accomplished using aconventional pitch-up manoeuvre initiated using the elevator 13 or vanes18 combined with throttling of the motor 24. Transition from hover tohorizontal flight is accomplished in an inverse manner.

When the aircraft 10 rolls for any reason, for example if disturbed by agust of wind or if rolled during a turning manoeuvre, the rolling momentdue to the increase in height gained by the centre of mass 29 willproduce a tendency to roll in the opposite sense so that the centre ofmass 29 may return to its lowest height. Hence, when disturbed by a gustof wind, the aircraft 10 will correct the induced roll straight awaywithout the need to use the vanes 18 or the elevator 13. In the case ofa turning manoeuvre, the aircraft 10 will self-right to its stablein-flight orientation as soon as the vanes 18 and the elevator 13 are nolonger deployed.

A second embodiment of the invention is shown in FIGS. 5 and 6. Thisembodiment broadly corresponds to the first embodiment, and so likereference numerals are used for like parts.

This alternative embodiment differs firstly in the design of the tailsection 11 b of the ring-wing 11 and the elevator 13. The ring-wing 11is truncated obliquely along a curve, so that it cuts the top of thering-wing 11 at a relatively sharp angle and then becomes progressivelyshallower to meet a ‘V’-shaped beaver-tail section 11 b. A moveableelevator 13 is recessed into each side of the ‘V’ and is hinged at itsforward edge. These elevators 13 work in tandem to control the pitch ofthe aircraft 10 or in combination to control the roll of the aircraft10.

This alternative embodiment also differs in that several of thecomponents of the aircraft 10 are housed in a compartment located withinthe lower front portion of the ring-wing 11. Although the compartment isnot shown explicitly, its position is shown generally by the referencenumber 30 in FIG. 5.

The person skilled in the art will appreciate that modifications can bemade to the embodiments described hereinabove without departing from thescope of the invention.

For example, rather than using the vanes 18 to compensate for the torqueproduced by the rotating propeller 15, a contra-rotating propeller couldbe employed. This also has the benefits of increasing the thrustproduced, provided the correct optimisation of the propeller blades isundertaken.

As an alternative to using styrofoam for parts of the aircraft 10, theycould be made from silica aero-gels which, advantageously, have very lowdensities but retain high structural strength. Composite structuresincluding lightweight honeycomb fillers contained within a thin shellcould also be used. Where the invention is embodied as a large aircraft,conventional aircraft materials could be used, for example the ring-wing11 could have a conventional structure of spars, struts and ribs. Whilsta profile that remains uniform around the ring-wing 11 is used in theabove embodiment, the profile could vary around the ring-wing 11. Forexample, to gain extra lift, an exaggerated camber may be used on thetop and bottom sections of the ring-wing 11, the camber being in thesame sense for both parts. Clearly, the profile of the ring-wing 11 mustvary between the top and bottom sections for the camber to be in thesame sense.

Furthermore, rather than operating the aircraft 10 by remote control,the aircraft may be autonomous, employing an auto-pilot for example.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

What is claimed is:
 1. An aircraft comprising: a fuselage; and apropulsion unit; wherein said fuselage is in the form of a ring-winghaving a substantially circular cross-section defining a duct with alongitudinally-extending central axis, said propulsion unit beinglocated within the duct and moveable aerofoils for controlling theaircraft in flight, the ring-wing fuselage being truncated obliquely atone end, such that a rear end of the aircraft, relative to a directionof horizontal flight, is truncated obliquely to form a ring-wing withopposed sides of unequal length, with a rear lower side extending beyonda rear upper side.
 2. An aircraft according to claim 1, wherein thering-wing fuselage is truncated by a planar slice through the ring-wingfuselage.
 3. An aircraft according to claim 1, wherein the ring-wingfuselage is truncated by a curved slice, the angle made with thering-wing fuselage being relatively shallow at the longer side of thering-wing fuselage and relatively steep at the shorter side of thering-wing fuselage.
 4. An aircraft according to claim 1, wherein atleast one of components and any payload are housed in a compartment thatis external to the longer side of the ring-wing fuselage.
 5. An aircraftaccording to claim 4, wherein the compartment is provided at an end ofthe ring-wing fuselage, that end being the front when in horizontalflight.
 6. An aircraft to claim 1, wherein components and payload arehoused in a compartment provided within the longer side of the ring-wingfuselage.
 7. An aircraft according to claim 1, wherein: at least one ofsaid aerofoils is a pitch controlling aerofoil; and the pitchcontrolling aerofoil is positioned to reside substantially flat with thelower side of the ring-wing fuselage when not deployed.
 8. An aircraftaccording to claim 7, wherein the pitch-controlling aerofoil is providedas an element of the longer side of the ring-wing fuselage.
 9. Anaircraft according to claim 7, further comprising two orthogonal vanes,wherein one vane provides aerodynamic lift in the same as thepitch-controlling aerofoil.
 10. An aircraft according to claim 1, whichis unmanned.
 11. An aircraft comprising: a fuselage in the form of aring-wing having a cross section defining a duct with a longitudinallyextending central axis; a propulsion unit located within the duct;moveable air foils located within the duct for controlling the aircraftin flight; wherein the ring-wing fuselage is truncated at a rearward endthereof relative to direction of air flow through said duct, formingopposed portions of said ring-wing fuselage which are of unequal length,such that when said aircraft is in horizontal flight, a lower portion ofsaid ring-wing fuselage extends farther rearwardly than an upper portionthereof.